Classical Genetics — Lecture I

1. Classical Genetics — Lecture I • Dr. Steven J. Pittler • VH375B • Office 4-6744 • Cell 612-9720 Suggested Reading: Lewis 2nd Edition Chapter on Mendelian Inheritance

2. Inheritance • Parents and offspring often share observable traits. • Grandparents and grandchildren may share traits not seen in parents. • Why do traits disappear in one generation and reappear in another?

3. Gregor Mendel: The father of modern genetics Combined • plant breeding • Statistics • Careful recordkeeping Described hypothesis of transmission of traits now considered the laws of inheritance

4. Mendel studied pea traits with two distinct forms

5. True breeding plants Plants which consistently have offspring with same trait as parent are true breeding plants.

6. Monohybrid cross • What happens when true breeding plants with two distinct forms of a trait are crossed? Progeny show only one form of the trait. The observed trait is called dominant. The masked trait is called recessive.

7. Test cross Test by crossing with a plant showing the recessive trait. Is a plant showing the dominant trait true-breeding or not? All tall offspring indicate parent is true-breeding Mixed offspring indicate parent is hybrid

8. Crossing hybrids to each other Hybrid parents show the dominant trait (tall). Offspring: • Dominant trait (tall) and • true breeding (1/4 total) • Dominant trait (tall) and • NOT true breeding (1/2 total) • Recessive trait (short) and • always true breeding • (1/4 total) Mendel concluded that among the hybrid parents the short trait (recessive) was hidden but not absent

9. Mendel’s data Crossed true-breeding plants differing at one of seven traits. Crossed hybrid offspring to each other (all show dominant trait). Counted offspring of hybrid crosses.

10. If you know the genotype of the parents, it is possible to determine the gametes and use a Punnett square to determine the phenotypic ratio among the offspring. • When a monohybrid reproduces with a monohybrid, the results are 3 : 1. • This ratio is used to state the chances of a particular phenotype. • A 3 : 1 ratio means that there is a 75% chance of the dominant phenotype and a 25% chance of the recessive phenotype.

11. Punnett Squares The genes from one parent go here. The genes from the other parent go here.

12. Punnett Squares F1 generation

13. Gamete Formation (sperm and eggs) • Because homologous pairs separate during meiosis, a gamete has only one allele from each pair of alleles. • If the allelic pair isTt, a gamete would contain either a T or a t, but not both. • Tt represents the genotype of an individual. • Gametes are represented by T or t.

14. One-Trait Crosses and Probability • Laws of probability alone can be used to determine results of a cross. • The laws are: • (1) the probability that two or more independent events will occur together is the product of their chances occurring separately, and • (2) the chance that an event that can occur in two or more independent ways is the sum of the individual chances.

15. In the cross of Tt x Tt, what is the chance of obtaining either a T or a t from a parent? Chance of T = ½, or chance of t = ½ The probability of these genotypes is: The chance of TT = ½ x ½ = ¼ The chance of Tt = ½ x ½ = ¼ The chance of tT = ½ x ½ = ¼ The chance of tt = ½ x ½ = ¼ The chance of tall plants (TT, Tt, tT) is ¼ + ¼ + ¼ = ¾ or 75%.

16. Law of segregation Why do traits “disappear” in one generation only to reappear in a subsequent generation? • Each plant has two distinct separable • units (alleles) for each trait inherited from • each parent. (sister chromatids that together make a chromosome) • Gametes contain ONE allele for each trait. • Only one version is observed in an individual. The unit (allele) does not disappear. It may be present but hidden.

17. Gene locus (locus = location)

18. Law of segregation

19. Alleles Mendel’s units (or “elementen”) are called alleles. • versions of the same gene or DNA sequence. • differ in DNA sequence at one or more sites.

20. Genotype indicates the combination of alleles present • Homozygous alleles are the same • Heterozygous alleles differ Phenotype indicates the trait observed. Terms distinguish the observed form and the underlying alleles present.

21. Phenotype Tall plant Short plant Genotype Homozygous dominant “tall-associated” alleles Heterozygous Homozygous recessive “short-associated” alleles Genotype and phenotype Abbreviation of genotype TT Tt tt

22. Modern terms for Mendel’s crosses • Mendel’s true-breeding plants were homozygous for the alleles of a trait. • Mendel’s hybrid plants were heterozygous for the alleles of a trait.

23. Wildtype most common version in the general population • wildtype phenotype • mutant phenotype • wildtype allele • mutant allele • most common phenotype • phenotype different from the • wildtype • most frequent allele • associated with the • common phenotype homozygous. • allele associated with the • mutant phenotype.

24. Law of segregation: the monohybrid cross Two heterozygous parents produce gametes with T or t allele equally frequently. Offspring genotypes 1/4 TT : 1/2 Tt :1/4 tt Offspring phenotypes 3/4 tall : 1/4 short

25. Most common Mode of inheritance indicates the patterns with which the mutant phenotype is associated. Autosomal recessive Autosomal dominant X-linked recessive X-linked dominant Y-linked (holandric) mitochondrial

26. Autosomal dominant inheritance • Heterozygotes exhibit the affected phenotype. • Males and females are equally affected and may transmit the trait. • Affected phenotype does not skip generations.

27. Autosomal recessive inheritance • Heterozygotes carry the recessive allele but exhibit the wildtype phenotype. • Males and females are equally affected and may transmit the trait. • May skip generations.

28. Autosomal dominant Autosomal recessive Comparison of autosomal dominant and autosomal recessive inheritance Males and females affected? Yes Yes Males and females transmit the trait? Yes Yes Trait skips generations? No Yes At least one parent of affected child must be affected? Yes No

29. Law of independent assortment • Two genes on different chromosomes segregate their alleles independently. • The inheritance of an allele of one gene does not influence which allele is inherited at a second gene.

30. Law of independent assortment

31. Independent assortment of two traits • In a dihybrid cross, parents with two differing traits are crossed. • Which allele is dominant? Heterozygous peas are round and yellow. Therefore round is dominant to wrinkled yellow is dominant to green

32. Two traits segregating independently

33. Pedigreessymbolic representations of family relationships and inheritance of a trait

34. Autosomal dominant inheritance of brachydactyly Heterozygotes exhibit the phenotype.

35. Autosomal recessive inheritance of albinismHeterozygotes carry the recessive allele but exhibit the wildtype phenotype

36. A a A a AA Aa aa Aa Ellen is not affected and cannot carry aa genotype Ellen Michael ? Ellen and Michael’s parents must be carriers. Genetic predictions Ellen’s brother Michael has sickle cell anemia, an autosomal recessive disease. What is the chance that Ellen’s child has a sickle cell anemia allele (a)? chance Ellen is a carrier = 2/3 chance child inherits sickle cell allele = 1/2 Overall chance child carries sickle cell allele from Ellen = 2/3 x 1/2 = 1/3